Various types of DSLs are collectively referred to as xDSLs, and xDSLs are a high-rate data transmission technology for transmission over a telephone twisted pair (that is, unshielded twisted pair (UTP). In addition to DSLs for baseband transmission such as an Integrated Services Digital Network DSL (IDSL) and a symmetrical high-speed DSL (SHDSL), xDSLs further include xDSLs for passband transmission. For an xDSL for passband transmission, a frequency division multiplexing technology is used to enable the xDSL and a plain old telephone service (POTS) to coexist over a same twisted pair, where the xDSL occupies a high frequency band, and the POTS occupies a baseband lower than 4 kilohertz (kHz). A POTS signal and an xDSL signal are separated by a splitter. Discrete multitone (DMT) modulation is used for the xDSL for passband transmission. A system that provides multiple xDSL connections is referred to as a DSL access multiplexer (DSLAM), and a system model of the DSL access multiplexer is shown in FIG. 1.
As an access node (AN), a DSLAM device can access multiple customer premises equipment (CPE), and on an AN side, there are multiple central offices (CO) in a one-to-one correspondence to the CPEs. Because of the principle of electromagnetic induction, multiple signals received by the DSLAM interfere with each other, which is referred to as crosstalk. Crosstalk between multiple signals includes near-end crosstalk (NEXT) and far-end crosstalk (FEXT). A process of forming the NEXT is shown in FIG. 2, where a downstream data stream sent by a CO1 to CPE 1 generates NEXT to a CO2 adjacent to the CO1, and an upstream data stream sent by the CPE1 to the CO1 also generates NEXT to the CPE2. A process of forming the FEXT is shown in FIG. 3, where a downstream data stream sent by a CO1 to CPE 1 generates FEXT to CPE 2 adjacent to the CPE1, and an upstream data stream sent by the CPE1 to the CO1 also generates FEXT to the CO2.
Energy of both NEXT and FEXT increases as a frequency band becomes higher. As shown in FIG. 4, in all existing xDSL technologies, such as asymmetric DSL (ADSL), ADSL2, ADSL2+, Very-high-bit-rate DSL (VDSL), and VDSL2, a frequency division duplex (FDD) technology is used on upstream and downstream channels, and in this case, impact of NEXT on system performance may be omitted and major impact comes from FEXT. However, as a frequency band used for an xDSL becomes increasingly wider, FEXT impacts transmission performance of the VDSL2 increasingly seriously. Currently, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has formulated a G993.5 standard to perform joint receiving and sending at a CO end using a vectoring technology, to cancel a FEXT signal.
DSL frequency spectrum resources are limited, and attenuation becomes larger as a line is longer; therefore, extension of frequency spectrum resources cannot improve performance. To resolve the foregoing problem, communications technologies of intra-frequency duplex such as a synchronized symmetric DSL (SSDSL) and trellis-coded modulation (TCM) in Japan are proposed. In an overlapped spectrum duplex (OSD) system, because upstream/downstream frequency spectrums overlap, NEXT is generated, as shown in FIG. 5. Because a very low frequency is used in the intra-frequency duplex technology and the frequency is usually below 1.104 megahertz (MHz), NEXT is not very serious. When a frequency band used becomes wider, impact from NEXT becomes increasingly serious, and performance cannot be further ensured.
In conclusion, although the Vectoring technology in the existing G.993.5 standard cancels FEXT, but the vectoring technology cannot resolve a problem of NEXT existing in an OSD system, which reduces performance of the OSD system. To reduce impact of NEXT, NEXT and FEXT need to be canceled in a joint cancellation manner.